This gets discussed quite a bit; but here is a recent article on the subject.
http://espn.go.com/nfl/draft2014/st...cks-difficult-project-teams-aware-high-stakes
But most of the teams that pick a quarterback early in this year's draft should know there's a probability their bet won't come through. Of the 137 quarterbacks taken in the first two rounds of the draft since the 1970 AFL-NFL merger, only 54 (39.4 percent) turned out to be five-year starters, according to Pro Football Reference.
http://espn.go.com/nfl/draft2014/st...cks-difficult-project-teams-aware-high-stakes